Lunar month

A lunar month is the interval between successive corresponding phases of the Moon, averaging about 29.5 days. Its length varies because the Moon’s orbit is not circular and Earth moves around the Sun.

Synodic month: The interval between successive identical lunar phases, averaging about 29.53 days. This is the precise astronomical name for the phase-to-phase interval meant by lunar month.
Anomalistic month: The interval between successive lunar perigees, averaging about 27.55 days. It tracks the Moon’s return to orbital closest approach, not the return of a phase.
Lunar calendar: A calendar system whose months follow lunar cycles. Its month boundaries are based on the phase cycle described by the lunar month.
Babylonian astronomy: The systematic observation and mathematical study of celestial bodies in ancient Mesopotamia. Babylonian records tracked lunar cycles and supported early calendar calculations.
Tides: The periodic rise and fall of sea level, driven mainly by the Moon’s and Sun’s gravity. Lunar phases modulate spring and neap tides through the changing alignment of Sun and Moon.
Sidereal month: The Moon’s orbital period relative to distant stars, averaging about 27.32 days. It is shorter because Earth’s motion around the Sun delays the return of the same phase.
Draconic month: The interval between successive passages of the Moon through the same node of its orbit, averaging about 27.21 days. Its node-to-node measure governs eclipse alignment rather than phase recurrence.
Lunisolar calendar: A calendar that follows lunar months while periodically aligning its year with the solar year. It uses lunar months but adds intercalary months to keep seasons aligned.
Metonic cycle: A period of 19 solar years that is nearly equal to 235 lunar months. It provided a way to reconcile lunar months with the solar year.
Spring tide: A tide with an unusually large tidal range occurring near new and full moons. These phases place the Sun, Earth, and Moon close to a line.

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